Picosecond dynamics of light induced optical nonlinearities was investigated in InN layers over wide range of temperature and excitation using complementary time-resolved pump-probe techniques. Picosecond light-induced transient grating (LITG) technique has been applied for investigation of nonequilibrium carrier diffusion and recombination in T=15K-300K range. Femtosecond differential transmission (DT) technique provided kinetics of induced bleaching and free carrier absorption in 1550-2300 nm spectral range. The measurement were carried out in two (2.3 μm and 0.6 μm thick) InN layers grown on sapphire with background electron density of 1.4×10¹⁸ cm^-3 and 4.7×10¹⁸ cm^-3.
LITG experiments provided novel features of non-equilibrium carrier recombination in highly excited InN (Δn>10¹⁸ cm^-3). The recombination rate was found strongly nonlinear and followed the dependence 1/τ_R∝B*(n0+N) which was nearly independent on temperature down to 40K. These features allowed us to suggest a trap-assisted Auger recombination being the dominant recombination mechanism in InN. The modelling provided a value of trap-assisted Auger recombination coefficient B*=8×10^-10 cm³s^-1.
DT kinetics at fixed pump energy and various probe wavelengths revealed the spectral range, where the induced bleaching was dominating (for wavelengths above the absorption edge at ~0.7eV). By increasing probe wavelength, we observed a gradual transfer from the optical bleaching (Δα<0) to the induced free carrier absorption (Δα>0 at λp>2260 nm) as well as the intermediate spectral region when these DT effects nearly cancelled each other. Measurements of free carrier absorption kinetics and determined carrier lifetimes at various excitation energy densities confirmed the inverse dependence of lifetime vs total carrier density. Moreover, the latter data provided a value of B*=4×10^-10 cm³s^-1, which is close to the one obtained by LITG technique.

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